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Milky Way spins faster, has more mass than thought

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  1. Jan 6, 2009 #1

    Redbelly98

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    Saw this on Yahoo! News; the title pretty much sums it up.
    http://news.yahoo.com/s/afp/20090105/ts_afp/usastronomy_090105234256

    My question for the regular astro PFers: what would be the sun's revolution period around the galaxy based on this new information?
     
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  3. Jan 6, 2009 #2

    marcus

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    Redbelly, these results were under embargo until 5 Jan, yesterday. I searched yesterday and today for something more technical than the press-release material and couldn't find.

    According to this report
    http://www.guardian.co.uk/science/2009/jan/05/galaxy-collision-space-milky-way
    the new estimate of the mass (including DM) is 270 billion solar mass. But this is similar to the estimates 200-400 billion that I have seen for some time!

    So until I see some more technical report I don't know what to think. Maybe the real news is that they have narrowed down the uncertainty. And that the new more precise estimate is higher than yesterday's LOW estimates. However it turns out, we can address your question about the orbital period. The Harvard-Smithsonian press release is here
    http://www.cfa.harvard.edu/press/2009/pr200903.html
    and it implies the orbit radius is 28,000 LY, so assuming circular the circumf is 176,000 LY
    and the estimated speed is 600,000 mph
    So we just type into google something like
    (176 000 lightyear) / (600 000 mph)
    and google will say
    (176 000 lightyear)/(600 000 mph) = 196 714 211 years
    So we can quickly say the orbit period is 200 million years.

    Indeed this is a tad shorter than what I remember from earlier. Back in 1970s I believe a common estimate was around 240 million years.
    A good question would be if this 270 billion solar is right, then does that refer just to the mass inside the sun's orbit? Because their methodology seems to be good for finding the sun's orbital speed and that would just give a handle on what's inside our orbit. If they are trying to include an estimate of the outer part of the disc, then how are they doing that? Maybe someone else here will clarify.
     
    Last edited: Jan 6, 2009
  4. Jan 6, 2009 #3

    Redbelly98

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    Doh! That info is also in the Yahoo article I linked...somehow I had glazed over it before.

    My notes have 250 Myr, based on google searches a year or 2 ago. I keep a list of time scales found in nature, from the Planck time on up to the age of the universe. The sun's orbital period is on my list, so when I read this article I realized I needed to update it.

    Thanks marcus!

    Mark
     
  5. Jan 6, 2009 #4

    marcus

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    Thanks for starting the thread!
    I found another estimate 914,000 kph (a.k.a. km/h)
    (this time in a BBC article)
    176000 lightyear/914000 kph
    Google says:
    "(176 000 lightyear) / (914 000 kph) = 207 821 117 years"
    so 208 million years.
     
  6. Jan 7, 2009 #5

    Redbelly98

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    And thanks for joining it.

    I'm interested in what numbers are given in the technical reports, with error bars. I imagine you'll see what those are eventually. The Yahoo article appears to make the all-too-common journalist error of too many sig figs after converting units:

    500,000 mi/hr becomes 804,672 km/hr
    600,000 mi/hr becomes 965,600 km/hr

    It's likely the BBC's 914,000 kph is believable, since they probably didn't need to convert units.

    And yes, I'm familiar with the Google calculator. It even recognizes "pi", so it can do the entire calculation:

    (2*pi*28 lightyear) / (914000 kph) = 207 737.503 years
     
  7. Jan 9, 2009 #6

    Redbelly98

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